1. Field of the Invention
The present invention relates to a process for forming an ultrafine particle film, especially an ultrafine particle film having both anti-reflection and antistatic functions, a light-transmitting plate and an image display plate formed by applying this technique, and a process for the production thereof.
2. Related Art
Films to reduce the reflectivity (anti-reflection films) capable of reflecting the visible light on a transparent plate surface have been long studied, and have been applied to lenses for cameras and ophthalmic glasses. At present, such films are used as an anti-reflection filter for reducing the reflected light on VDTs (visual display terminal). A variety of anti-reflection films have been proposed, and those mainly used now are multi-layered films and heterogeneous films.
A multi-layered film has a structure in which a material having a low refractive index and a material having a high refractive index are alternately stacked to form at least three layers. Its anti-reflection effect is a synergistic effect produced by the optical interference function of each layer. Multi-layered films are discussed in Physics of Thin Films, 2 (1964), pp. 242-284.
A heterogeneous film having a refractive index distribution in the film thickness direction is generally formed by rendering a transparent plate surface porous.
Apl. Phys. Lett., 36 (1980), pp. 727-730 discusses a method of reducing the reflectivity in which a heterogeneous film is produced by forming an insular metal deposition film on a glass surface and forming a fine uneven surface by sputter etching.
On the other hand, in a cathod ray tube, there are required not only formation of an electrically conductive film for preventing electrostatic charge on glass surface but also use of devices for preventing reflection.
Meanwhile, it is known that the front panel surface (image display plate) of a cathode ray tube such as a Braun tube is electrostatically charged. The reason therefor is as follows. Aluminum is generally deposited to form a thin and uniform film 84 on a phosphor 83 applied to an inner surface 82 of a Braun tube 81 as shown in FIG. 8. In the application of a high voltage to the aluminum film 84, an electrostatic charge occurs on a front panel 85 of the Braun tube owing to electrostatic induction when the high voltage is applied and cut off.
For example, JP-A-61-51101 discloses a method for preventing both electrostatic charge and reflection on such a display tube surface. In this method, first an electrically conductive film is formed on a glass substrate by a physical vapor phase method or a chemical vapor phase method, such as vacuum deposition, sputtering, etc., and then anti-reflection film is formed thereon.
The above prior arts entail high cost because of necessity for high-precision control of the film thickness, and moreover the film-forming method used in them is limited to a sputtering or vacuum deposition method. Thus, application of these prior art methods to a substrate having a large surface area is substantially impossible owing to the restrictions on the apparatus which are inevitable in these methods.
The anti-reflection films formed by the above methods are basically of a structure in which the materials differing in refractive index are deposited in layers on a glass surface, and reflection is prevented by an optical interference function of each layer. For facilitating understanding of the anti-reflection mechanism, a most simple single-layer deposited film is considered here. When a glass surface having a refractive index of Ng is coated with a material having a lower refractive index than glass, Nf, to a thickness of d, the reflecting behavior of the light incident on this surface can be determined from the Fresnel's formulas, and the reflectivity R is given by the equation 1:
Equation 1 ##EQU1## Here, it is assumed that there exists the relation of the equation 2:
Equation 2 ##EQU2## wherein d is layer thickness, and .lambda. is light wavelength.
From the above equation, R=0 when Nf=.sqroot.Ng. This signifies a state where there is no reflection of light with a wavelength of .lambda.. Since Ng is 1.52 for the most common soda glass, coating of the glass with a material with Nf=1.23 gives an ideal anti-reflection film at a wavelength .lambda. which is decided according to the film thickenss d. However, there is not yet any available material having such a low refractive index, and among the materials usable at present, magnesium fluoride (MgF.sub.2) with Nf=1.38 is the material having the lowest refractive index. In this case, the reflectivity R=1.3%. As apparent from the equations 1 and 2, anti-reflection conditions for a single-layer film are set for a specific wavelength .lambda., and the reflectivity increases around this specific wavelength .lambda.. Therefore, in order to reduce the reflectivity in the whole region of visible light (400-700 nm), it has been necessary to laminate the materials with different refractive indices to form a multi-layer structure while strictly controlling the film thickness. Surface reflection can be reduced also by using a heterogeneous film having a refractive index distribution in the film thickness direction. In case the glass surface has such an unevenness as is illustrated in FIG. 10, the refractive index (nF(x)) can be represented by the equation 3 when the coordinate in the layer depth direction is expressed by x:
Equation 3 EQU nF(x)=ng.times.V(x)+(1-V(x)).times.n.sub.0
wherein ng is the refractive index of glass, V(x) is the volume occupied by glass at x, and no is the refractive index of air.
In this case, the refractive index varies discontinuously at the interface between air and film and at the interface between film and glass substrate as shown in FIG. 11. Therefore, when the refractive indices at these points are taken as n.sub.1 and n.sub.2, respectively, the reflectivity R of this layer is represented by the equation 4:
Equation 4 ##EQU3##
The reflectivities determined from the above equation at n.sub.0 =1.0 (refractive index of air), n.sub.1 =1.1, n.sub.2 =1.47 and ng=1.53 (refractive index of glass) with visible light wavelengths are graphically shown in FIG. 12. It will become clear from this graph that the lowest reflectivity can be obtained when the surface roughness is around 100 nm (0.1 .mu.m). It is, however, difficult to provide an unevenness of such a size regularly on a glass surface, and much time is required for forming such an unevenness even by etching. The present inventors have previously proposed a film having low reflection characteristics comparable with a three-layer deposited film by forming said unevenness with ultrafine particles. In case an uneven film is formed with ultrafine particles, the reflection characteristics are basically represented by the equation 4, but since a thin binder layer 2 is formed between the ultrafine particles 4 and the surface of the substrate 3 as shown in FIG. 13, it is necessary to distinguish between the thin layer 2 and the ultrafine particle layer 4. The ultrafine particle surface layer can be diagrammatically represented by a model shown in FIG. 14. In this case, when ng (refractive index of glass, 1.53) in the equation 4 is replaced by ns (double refractive index of binder and ultrafine particles, 1.47), the following equation 5 is given:
Equation 5 ##EQU4##
Assuming n.sub.0 =1.0, n.sub.1 =1.10, n.sub.2 =1.38 and ns=1.47, Ra is about 0.19% at .lambda.=550 nm.